As the use of various energy sources increase, there are increasing demands to obtain energy from renewable resources, and particularly from renewable resource scraps or residues thereof. One such energy source which is receiving increasing attention is the burning of wood and particularly wood residues, i.e., bark and wood scraps remaining from wood processing or forest wastes, to produce steam in boilers for process heat in factories, for space heating, and for generation of electricity.
Wood residue fuels are characterized by having a relatively high percentage of volatile matter, ranging from approximately 75% to 80% volatiles by weight, and often a relatively high moisture content, ranging from about 5 to 65% by weight. In addition, wood residues include less solid carbon per pound of fuel than coal.
The conventional methods of direct combustion of wood residue fuels for industrial use have utilized either the Dutch oven type of design, a suspension burning design or a spreader-stoker design. Of these designs the major portion of newly constructed boilers are of the spreader-stroker configuration. Presently operating wood residue spreader-stoker boilers, which generally use a fuel bed depth of about 1 inch or less and about 6 or 7 pounds or underfire air per pound of fuel, have been troubled by relatively high emissions of ash and unburned char which is carried out of the combustion chamber. High particulate carryover rates generally result in difficulties in meeting emission standards unless extensive pollution control devices are connected to the boilers. Further, the high carbon carryover rates result in decreased combustion efficiency since the fuel value of the carbon is not utilized in the boiler or furnace.
The burning of wood residue fuels in such furnaces or boilers have required rather elaborate and expensive particulate collection systems to meet emission standards for continued operation. The collection system costs are normally a large portion of the capital costs in building one of these energy source systems. The economics of burning of wood residue fuels may thus be governed by the cost of emission cleanup equipment.